The production of carbon electrodes is known to include the processing of coke from petroleum residues and the subsequent combination of the coke with a binder resin to provide the electrode. Coke, a solid infusible material is conventionally obtained via coking processes wherein heavy petroleum oils are heated for long periods in vessels until the coke layer builds up in the vessels. Depending upon the petroleum used as well as the processing steps and conditions employed, different types of coke are obtained. For the manufacture of carbon electrodes for use in the metal industry, several qualities of coke are recognized, the highest of which is called needle coke.
Needle coke has the lowest CTE of coke formed from petroleum hydrocarbons and is the most desirable for use in steel making. The CTE of needle coke should be less than 3.5.times.10.sup.-7 to insure a low value in the resulting electrode inasmuch as existing processes have not produced electrodes with lower values than that possessed by the coke. The preparation of needle coke from petroleum is known and has been described in U.S. Pat. No. 2,775,549. In addition to disclosing several methods for producing needle coke, the patent further discloses the production of electrodes therefrom utilizing a coal tar pitch binder. Several electrodes reported in the patent according to the disclosure were measured and found to have CTE values ranging from 19 to 32.times.10.sup.-7. Although these were good values for that time, by recent standards, such electrodes would only be suitable for the manufacture of aluminum, not steel.
With respect to binder pitch for these electrodes, both coal tar and petroleum pitches have heretofore been employed. A process for producing petroleum binder pitches is set forth in U.S. Pat. No. 3,355,377. The pitch is obtained in the presence of an activated carbon catalyst and is derived from the bottoms fraction after refluxing a petroleum hydrocarbon. The use of such binders in the manufacture of electrodes is stated in the patent and although the preparation of several electrodes utilizing coke aggregate having a mesh size of 4 to 200 is reported, no CTE values appear.
A U.S. patent directed toward petroleum derived resins is U.S. Pat. No. 3,265,093, owned by the assignee of interest herein. The patent discloses the impregnation of porous tubular members to help the latter survive in underground installations. Impregnation is with what is described as a petroleum derived quinoline soluble oxygenated condensation product of a distilled clarified cycle oil extract. Clarified cycle oil, in turn, refers to the material obtained as bottoms in fractionating the output of a catalytic cracking process. There is no disclosure of this impregnant being used as a binder for electrodes.
The use of coal tar and petroleum pitches as a binder for carbon electrodes has been studied by L. F. King and W. D. Robertson and is reported in an article entitled "A Comparison of Coal Tar and Petroleum Pitches as Electrode Binders," Fuel (London) 1968, 47(3), pp. 197-212. The authors basically acknowledge that coal tar pitch has been well used as a binder for petroleum coke in the preparation of carbon electrodes for aluminum manufacture. After indicating that petroleum pitches have not generally given favorable analytical data as compared to coal tar pitches, they report that petroleum thermal tars were up-graded in their work by heat soaking operations. The preparation and testing of electrodes is also discussed in their report although only density and compressive strengths are included, not CTE values. Based upon the authors' standard of electrodes that are suitable for aluminum production, those skilled in the art can appreciate that such electrodes containing petroleum pitches would not be suitable for steel manufacture where lower CTE values are a necessity.
Thus, while it is known to manufacture ultra high grade electrodes for steel making processes having low CTE values, the art of which we are aware has only appreciated that needle coke is indispensible as a component for those electrodes. Little, if anything, has been done to derive a binder material that will improve the CTE value of the resulting electrode.